1. Field of the Invention
The invention relates to a high torsional stiffness low axial deflection stiffness flexible drive shaft for rotating recordable media, such as magnetic tapes or discs or video disks.
2. Discussion of the Related Art
There are at least three options for coupling a drive motor to one or more tape reels in a tape drive. The general problem of off-center drive exists in all cases, but is handled in different ways. The first option is a data cartridge which uses a belt drive. Although the belt inside the cartridge has more function than just solving the off-center drive problem, it is nevertheless one way to solve the problem. The second option, which is used in all audio and video cassette systems is to allow the tape reel to "float" inside the cartridge. The tape reels in such systems do not rotate on axes that are rigidly attached to the cartridge. The reels can move within a certain range to align with the drive axes. Usually the engagement method has some kind of self-centering device, such as spider gears, or the reel is allowed to wobble a little (audio and video tape cassettes). The third option is to have the tape reels rotate on axes that are rigidly connected to the cassette housing, and allow the drive shafts axes to adjust to the cassette reels. The advantage of this kind of system is that a precision tape path can be defined totally inside the cassette for excellent guiding, low debris generation, etc. Note that the first option also has the tape path totally defined inside the cassette.
In the third option, each reel includes an axle with a bearing that is designed to have a close fit; however the axle will rock slightly if the tape driving mechanism ("tape drive") is not aligned with it. This rocking motion results in an unreliable tape path which, in turn, causes low quality recording or playing of the magnetic tape.
Some magnetic tape cassettes have a gear connected to each reel axle. In a like manner, the tape drive has a pair of drive gears, each drive gear being engageable with a respective reel axle gear to implement the tape driving function. In addition to low quality tape performance, misalignment between the drive system and the reel axles further results in improper meshing of the gears. This improper meshing causes wear of the gears and creates debris which can adversely affect system performance.
The off-center drive problem is a classic mechanical problem, with numerous solutions existing in the prior art. For example, automobile drive trains often use a pair of universal joints and a telescoping joint to solve the problem. Farm implements and other heavy machinery commonly use elastic joints to accommodate offsets. Precision equipment often employ bellows couplings, Oldham couplings, and spider couplings. None of these prior art teachings solves the cassette offset drive problem. A tape drive for a magnetic tape cassette must have high torsional stiffness and low axial deflection stiffness, and yet must be capable of supporting a thrust load. The high torsional stiffness is required to rotate the cassette reel and the low axial deflection stiffness is required to prevent the exertion of tipping torque on the reel axle. Tipping torque that is too large is what causes the reel to rock back and forth. The difficult problem is in providing low axial deflection stiffness and at the same time supporting the necessary thrust load to make driving engagement. The tape drive must also deliver a constant velocity with low drag. This is necessary in order to obtain a reliable tape path for recording or playing the magnetic media. Further, exacting engagement between the tape drive and each cassette drive axis is necessary to minimize gear wear and debris generation.
Many of the aforementioned problems associated with magnetic tape drives also apply to magnetic or video disk drives. These drives receive magnetic or optical disks. Such disks normally have a central aperture which engages a drive system grabette, the grabette being powered for rotating the disk. Alignment between the disk drive and the central axis of a disk is necessary to prevent any rocking motion which will impact on any play or record path.